Proficiency in vs proficiency at - Which should be used in the provided sentence?

Question

1. His skills include proficiency in using various programming languages.
2. His skills include proficiency at using various programming languages.

A post on Quora suggests the latter sentence is more grammatically accurate than the former, saying, "Proficient in is generally used when discussing a subject area: proficient in science, proficient in auto mechanics. Proficient at is used with a specific activity: proficient at swimming, proficient at building houses."

However, Grammarly suggests proficiency in is correct in the first sentence and that using proficiency at instead of it in that sentence would be grammatically incorrect. Which of the two sentences is grammatically correct? Are they both fine?

Answer 1

You would use proficiency in, when talking about a field. fx. proficiency in mathematics. And you would use proiciency at, when describing an action. fx. proficiency at solving equations.

In your specific case, using various programming languages sort of covers both uses, as you are covering a broad field ith the term various programming languages.

If you really want to be technical about it, proficiency at, is the most correct. Since 'using' is refering to an action. So it is proficiency in acting on/in/with a field (various programming languages).